1. Field of the Invention
This invention relates to induction motors. More particularly, this invention relates to speed control of induction motors employing switchable capacitors connected in series with the auxiliary/start winding of the motor.
2. Description of the Background Art
Conventional induction motors typically comprise a main winding and an auxiliary/start winding. A capacitor and a resistor are connected in series with the auxiliary winding. Voltage from an alternating current (AC) power source is applied across the windings by means of a switch which, for high speed operation, applies voltage across the main and auxiliary windings connected in parallel and, for low speed operation, applies voltage across only the main winding. Additional speed control can be obtained by switching in different capacitors into the auxiliary winding. U.S. Pat. No. 4,670,699 discloses various speed control techniques for induction motors wherein the capacitors in the auxiliary winding are selectively switched in and out of the auxiliary winding so as to achieve high, medium, and low speed operation.
As reflected in U.S. Pat. No. 4,670,699, the resistor connected with the capacitor(s) in the auxiliary winding provide a means for safely discharging the capacitor(s) when changing from one speed to another. Indeed, it has been known that without such a resistor, the capacitor(s) will not be safely discharged during switching thereby resulting in increased arcing during switching. Moreover, the need for such a resistor unnecessarily increases the manufacturing costs.
Therefore, it is an object of this invention to provide an induction motor speed control apparatus and method which overcomes the aforementioned inadequacies of the prior art controls and provides an improvement which is a significant contribution to the advancement of the induction motor art.
Another object of this invention is to provide speed control for induction motors by switching in various capacitors in the auxiliary winding of the motor and by providing means for discharging such capacitors during switching without employing a resistor connected in the auxiliary winding.
Another object of this invention is to provide speed control for induction motors wherein a switch is provided for selectively switching in various capacitors in series with the auxiliary winding of the motor for achieving multi-speed operation of the motor, the switch including means for disconnecting the power source from the in-use capacitor and, during such disconnection, momentarily connecting such capacitor to the main winding, allowing such capacitor to discharge through the main winding and the auxiliary winding, and then connecting the power source to one or more of the other capacitors or reconnecting the power source to such capacitor in parallel with one of the other capacitors.
Another object of this invention is to provide speed control for induction motors as described hereinabove, wherein the switch comprises a rotary switch having indexed positions corresponding to the "off" position and each position of speed control (e.g. "low", "medium" and "high"), configured in such a manner that each of the in-use capacitors in the auxiliary winding is momentarily disconnected from the power source and connected to the main winding as the switch is rotating from one of such positions to another.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.